1. Field of the Invention
The present invention relates to display panel devices, display devices, and control methods thereof, and in particular, to a display panel device and a display device using current-driven luminescence elements and a control method thereof.
2. Description of the Related Art
Image display devices using organic electro-luminescence (EL) elements are known as image display devices using current-driven luminescence elements. The organic EL display devices using self-luminous organic EL elements are best suited to make thinner devices because such organic EL elements do not require backlights conventionally required for liquid crystal display devices. In addition, having no limitation on viewing angle, the organic EL display devices are expected to be practically used as the next-generation display devices. Further, in the organic EL elements used in the organic EL display devices, luminance of each luminescence element is controlled according to current value of current flowing therein. This differs from liquid crystal cells each of which is controlled according to voltage to be applied thereto.
In a usual organic EL display device, organic EL elements which serve as pixels are arranged in a matrix pattern. An organic EL display device is called a passive-matrix organic EL display device, in which organic EL elements are provided at intersections of row electrodes (scanning lines) and column electrodes (data lines) and voltages corresponding to data signals are applied between selected row electrodes and the column electrodes to drive the organic EL elements.
On the other hand, switching thin film transistors (TFTs) are provided at intersections of scanning lines and data lines, connected to gates of driving elements, and turned ON through selected scanning lines to allow data signals to be provided to the driving elements via signal lines. An organic EL display device including organic EL elements driven by such driving elements is called an active-matrix organic EL display device.
In the passive-matrix organic EL display device, the organic EL elements connected to the row electrodes (scanning lines) produce luminescence only in a period during which the connected row electrodes are being selected. On the other hand, the active-matrix organic EL display device allows the organic EL elements to keep producing luminescence until next scanning (selection); and thus, there is no reduction in luminance of display in the active-matrix organic EL display device even when the number of scanning lines increases. Accordingly, the active-matrix organic EL display device can be driven at a low voltage, thereby consuming less power. However, in the case of the active-matrix organic EL display device, due to variations in characteristics of driving transistors, even when the same signal is applied, luminance of the organic EL elements is different for each pixel, thereby causing a problem of variations in luminance.
In order to address this problem, Patent Reference 1 (Japanese Unexamined Patent Application Publication No. 2008-203657), for example, discloses a method of compensating for pixel-to-pixel variations in the characteristics using a simple pixel circuit, as the method of compensating for variations in luminance caused due to the characteristic variations of the driving transistors.
FIG. 14 is a diagram showing a circuit configuration of a pixel unit included in a conventional display device disclosed in Patent Reference 1. A display device 500 shown in FIG. 14 includes a pixel array unit 501, a horizontal selector 503, a light scanner 504, and a bias scanner 505. The pixel array unit 501 includes pixel units 502 arranged rows and columns.
The pixel unit 502 is configured with a simple circuit which includes: a luminescence element 508 having a cathode connected to a negative power line 512; a driving transistor 507 having a drain connected to a positive power line 511 and a source connected to an anode of the luminescence element 508; a capacitor 509 connected between a gate and the source of the driving transistor 507; an auxiliary capacitor 510 connected between the source of the driving transistor 507 and a bias line BS; and a sampling transistor 506 which has a gate connected to a scanning line WS and selectively applies a video signal from a signal line SL to the gate of the driving transistor 507. The following describes luminescence operation of the pixel unit 500.
The light scanner 504 supplies a control signal to the scanning line WS, and the horizontal selector 503 supplies reference voltage Vref to the signal line SL. With this, a correction operation is performed whereby voltage corresponding to threshold voltage Vth of the driving transistor 507 is held by the capacitor 509. Then, following this, a writing operation is performed whereby signal potential Vsig of the video signal is written to the capacitor 509.
The bias scanner 505 changes, before the correction operation, potential of the bias line BS and applies coupling voltage to the source of the driving transistor 507 via the auxiliary capacitor 510. By doing so, the bias scanner 505 performs a preparatory operation whereby voltage Vgs between the gate and the source of the driving transistor 507 is initialized to be greater than the threshold voltage Vth.
The pixel unit 502 negatively feeds the drain current of the driving transistor 507 back to the capacitor 509 in the writing operation of the signal voltage Vsig, so that correction is performed on the signal voltage Vsig according to the mobility of the driving transistor 507.
FIG. 15 is a chart showing operation timing of the conventional display device disclosed in Patent Reference 1. FIG. 15 shows operations of the display device for a single pixel row. A single frame period includes a non-luminescence period and a luminescence period. Further, in the non-luminescence period, the correction operations are performed on the threshold voltage Vth and mobility β of the driving transistor 507.
First, at time T1, when the frame period starts, a short control pulse is applied to the scanning line WS, causing the sampling transistor 506 to be in an ON state temporarily. Here, the potential of the signal line SL is reference voltage Vref; and thus, the reference voltage is written to the gate electrode of the driving transistor 507. This causes Vgs of the driving transistor 507 to be equal to or less than Vth, cutting off the driving transistor 507. This causes the luminescence element 508 to be in a non-luminescent state, and from time T1, the display device 500 enters a non-luminescence period.
Next, at time T2, a control signal pulse is applied to the scanning line WS so that the sampling transistor 506 is turned ON.
At time T3 which is immediately after time T2, the bias line BS is changed from high potential to low potential. This decreases the potential of the driving transistor 507 via the auxiliary capacitor 510. As a result, the relationship becomes Vgs>Vth, turning ON the driving transistor 507. Here, the luminescence element 508 is reversely biased. This does not allow current to flow through, and increases the source potential of the driving transistor 507. When Vgs becomes equal to Vth, the driving transistor 507 is cut off, and the correction operation of the threshold voltage is completed.
Next, at time T4, the potential of the signal line SL is changed from the reference voltage Vref to the signal voltage Vsig. Here, since the sampling transistor 506 is in a conductive state, the gate potential of the driving transistor 507 is Vsig. At this time, the luminescence element 508 is in a cut-off state initially; and thus, discharge current Ids that is drain current of the driving transistor 507 flows only through the capacitor 509 where the electrical discharge accordingly starts. After this, by time T5 at which the sampling transistor 506 is turned OFF, the source potential of the driving transistor 507 increases by ΔV. In this way, the signal potential Vsig is added to Vth and written into the capacitor 509. At the same time, the voltage ΔV used for mobility correction is subtracted from the voltage held by the capacitor 509. The above period from time T4 to time T5 is a signal writing period and also is a mobility correction period. As the Vsig increases, the discharge current Ids also increases, thereby increasing the absolute value of the ΔV as well.
FIG. 16 is a graph showing characteristics of discharge current of the capacitor in the mobility correction period. The horizontal axis represents elapse of time after writing of the signal voltage Vsig, that is, elapse of time after time T4, and the vertical axis represents discharge current values. As described above, when the gate potential of the driving transistor 507 is changed from the reference voltage Vref to the signal voltage Vsig at time T4, the discharge current Ids plots discharge curves, such as A1, B1 and C1, in accordance with the magnitude of Vsig. Here, A1 and A2 are discharge curves of the driving transistors in the case where the same magnitude of Vsig is applied to the gates of these driving transistors although these driving transistors have different characteristic parameters of the mobility β. The relationships between B1 and B2, and between C1 and C2 are identical to the relationship between A1 and A2. It can be seen from these discharge curves that, even when the same signal potential is applied, the initial values of the discharge current Ids are different if the characteristic parameters for the mobility β are different; however, the discharge current Ids become almost identical to each other with the elapse of discharge time. For example, the discharge current Ids of A1 and A2 become almost identical at time a, the discharge current Ids of B1 and B2 become almost identical at time b, and the discharge current Ids of C1 and C2 become almost identical at time c. To be more specific, even when the pixel array 501 includes the driving transistors having different characteristic parameters of the mobility β, the drain current of the driving transistor 507 is caused to be discharged, while the gate bias is applied such that the luminescence element 508 does not produce luminescence in the above-mentioned mobility correction period. Accordingly, the correction can be made, with consideration given to the variations in characteristics of the mobility of the driving transistors.
Next, at time T5, the scanning line WS transitions to low level side, turning OFF the sampling transistor 506. This separates the gate of the driving transistor 507 from the signal line SL. At the same time, the drain current of the driving transistor 507 starts to flow through the luminescence element 508. After this, Vgs is maintained constant by the capacitor 509. The value of Vgs here is obtained by correcting the signal voltage Vsig using the threshold voltage Vth and the mobility β.
Lastly, at time T6, the potential of the bias line BS is changed from low back to high, and then the next frame operation is made ready.
As described so far, the display device 500 disclosed in Patent Reference 1 suppresses the variations in luminance caused due to the variations in the threshold voltage Vth and in the mobility β.